Split side band amplifier circuit



Aug. 13, 1940. w. H. wlRKLl-:R

SPLIT SIDE BAND AMPLIFIER CIRCUIT Filed Aug. 4, 1938 3 Sheets-Sheet l Aug. 13, 1940. w. H. wlRKLER i SPLIT SIDE BAND AMPLIFIER CIRCUIT ATTORNEY Filed Aug. 4, 1938 Aug. 13, 1940- w. H. WIRKLER SPLIT SIDE BAND AMPLIFIER CIRCUIT Filed Aug. 4, 1938 3 Sheets-Sheet 5 INVENTOR. www

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Patented Aug. 13, 1940 lsrlirr SIDE BAND AMPLIFIER crncprr Walter Wirkln'ceaar Rapida Iowa, assignmto Collins'RadioCompany, CedarRapids, Iowa,`

a 'corporation of Iowa Application Augusta 193s, semaine. 223,096

4 claims.' (ci. 25o- 17) My invention relates broadlyztoan amplifica.- tion system for double vside band waves ofythe suppressed carrier type, .and more particularly to a split side band amplier forsets :ofv double `5 side band waves in phase quadrature. .An amplification system ofthis'type is` applicable, for example, in a radio transmission. system such as shown in a copending application 'by John F. Byrne, Serial No. 215,465, filed June 23, :1938;for Broadcasting system. Another application ofthe circuit of my invention isfor the amplification of single side'band waves for use in single'iside band suppressed carrier transmission systems of the type shown'inmy copending application Serial No..l76,670, led .November 26, 1937, for VSingle side'band transmitter.

One of the objects of my invention is to pro- 'vide side band amplification in tubes of one-half 'the power rating ordinarily required for. such purpose. i :y Another object of my 'invention is .to provide split side band amplification," that `is, separate amplification of superior and inferior side bands, yin order to produce increased. efficiency of amplication.

Still another object of my invention is to proivide an amplification system for .split side Vband circuits adapted for single'or double side` band systems. i l

A further object of my invention isthe pro'- vision of means for using the bridge circuit principle in splitting double'side band currents into two single side band currents and amplifying 'each of the single side band-currents in a separatefamplifier tube.

Another'object of my inventionis the provision of means for using thev bridge circuit principle in combining the single side band currents after ampliiication to produce the double side band cur- -k rents required. i

A still further object of my invention is to'pro- Vide a balanced bridge circuit for deriving single side band currents from setsof double side band components invphase quadrature, vfor transmission vexclusive of the other side band component.

Other and further objects o f my Ainvention lreside lin the circuits and arrangements herein disclosed` and hereinafter more fully -rdescribed with referenceto' the accompanying drawings, in which: 'l

Figure'l is a block diagram showing the basic arrangement `of elements in the splitA side band amplification system4 vof lmy invention; lig'.4 f2 is a schematic diagram ofy the circuitconnections more vparticularly involved in the split sidefbarid vviny` Figgl, and which are of theiorm:

'amplifier circuit; and Fig. '3 is aschematicdiagram of a modified form of the split side band amplifier, showing the transmission of one only of the side band components. n

The block diagram, Fig. l, shows the modulation and side band amplifierr portions of a transmitter embodying my invention of split side band amplication, The' source of carrier energyat I includes an oscillator and amplier., and the carrier energyis fed on the orielfiandjthrough .phase shifting network 2 to balanced modulator 3, and on the other hand directly tobalaned vmodulator', the waves in-thefdiierent rmodulatorsv being in lphase quadrature. ,Thel source of jsignalmodulations, lsuch as a program lineat l2, ,delivers modiilationjenergy to the, balancedumod'- ulators 3 and @through phase shiitingnetworks 5 andl. respectively. ,the modulation waves .inthe different modulators being out-of'phase. fllfhe carrier energy is suppressed in the balanced mod ulators and double side lbands are present in the outputs thereof, 90 out of phase in the separate circuits.' .y l

The system of my invention broadly contem- Iplatesthe division of theside bands in each cutput -into the superior and inferior bands, com'- bining both superior and both inferior components in the separate outputs, and'amplifying the single side bands, afterwards recombining superior and inferiorjside bands to produce double side band waves for radiation, `phase relations being adjusted in the amplier output. The division or splittingfof'the double. `side 'band outputs of the modulators is effected in a bridge circuit 1 adjacent arms of-wliich are constituted by the input circuits of 4linear ampliers j8 and 9, each operative on only one lsideeband. The recom# vbining of superior and inferior sidebands is effected in another bridge circuit I0, adjacent arms jef which feed double sideband energy lto the radiating system. :Phase relationsin' any particu- 'lar system may bevfurther adjustedor modified by a phase shifting network 'Il in'circuit 'from oneof the output arms of the-bridge circuitl. In the broadcasting systemdescribed in; the John E.V Byrne application, Serial `l\To. 215,465, supra, `in combination with )otherl factors,- two doubleside band antennacurrents are required, f

`such asfare obtainable from the system`shown 'quency factorsrespectively. 1In' thel Byrne ap'- the case in Fig. 1 above described in the absence of carrier'phase shifting network 2. These currents, representing double side band components, are separately amplied in the Byrne circuit and applied to the'antenna system. In the system of my invention, however, the currents are generated in somewhat different form which permits split side band amplification and attendant improved operation. Because of the difference in carrier excitation phase for modulators 3 and 4, Fig. 1, the outputs thereof are, respectively:

The currents I3 and I4 are split into two portions in bridge circuit "I, the components in the input to amplifier 8 being added while the components in the input of amplifier 9 are subtracted, thus:

and

:Il sin wt sin pt And similarly, the current in the other branch I4 at the output of bridge I0 is Phase shifting network II shifts the phase of both side band currents by 90, giving @sin @emersa (w+/1m and the output to the antenna line` I4 is therefore z`1=I1 sin wt cos pt. Thus i1 and i2 as supplied to the antenna system are of the 4form required for radiation in the Byrne broadcastling system.

While a rather complicated network is ordinarily required to shift the phase of a band of frequencies by equal amounts, an ordinary network designed to shift the phase of a single frequency will be entirely adequate for phase shift network II. The reason for this is the relatively narrow band of frequencies involved. For example: if the carrier frequency is 1000 kilocycles and the highest audio modulation frequency used is 10 kilocycles, the upper side band frequency will be 1010 kilocyclesand the lower side band frequency will be 990 kilocycles. In

,other words, the diierence between the highest and lowest frequencies is only a matter of 2%.

The economy of this method of generating the currents i1 and i2 is evident from Equations 1 and 2. That is, the maximum amplitude of the currents I1 and Is is only one-half that of the currents i1 and i2. For a constant plate voltage on the side band amplifiers, for example, this means that the two side band ampliilers of this system need have only one-half the peak voltampere rating required of side band ampliers which amplify lthe currents i1 and I2 directly. Further, for a pure tone modulation, linear am pliers 8 and 9 carry only a single radio irequency current. In the case of 100% modulation by a pure tone, for example, these linear ampliners would operate at a plate efficiency of approximately 67%. In the case of linear ampliiiers carrying double side band current, however, the average power output can be only onehalf the peak power output capability of the tube, because the envelope of the radio frequency current wave is in the form of a half sine wave. The result is that the eiciency of such a linear amplifier is only 53% instead of 67% for 100% modulation with a pure tone. The increase in eciency made possible by the use of this system is not particularly important because in ordinary broadcasting, modulation is with complex tones, and much of the time is considerably less than 100%. The reduction in tube capacity required is quite important, however,

`especially'inthe case of high powered radio transmitters; for example, a 500 kilowatt transmitter using the Byrne system of broadcasting and requiring eight tubes for the Byrne circuit, Would, by the use of the split side band amplifier circuit of my invention, employ only six tubes, and the proportionate saving in annual tube cost may be seen to be appreciable.

Fig. 2 is a schematic diagram of the modulation and split side band amplifier circuits of this invention, in which block I indicates the conventional oscillator and amplifier portion of the transmitter. Radio frequency current is supplied to coil L1 and induces carrier frequency voltage in coil L2 of phase shift network 2. Condensers C1 and C2 and coils L3 and L4 complete the phase network which supplies excitation voltage 90 degrees out of phase to the balanced;

modulators at 3 and 4. In modulator 3, V1 and V2 are the tubes of the balanced modulator, A1 is a filament current source, C3, C12 and C12 are blocking condensers, H1 is a radio frequency choke, and R1 is a grid leak resistor. The grids of V1 and V2 are connected in parallel, and the plates are connected in push-pull to the tuned output circuit, consisting of condenser C4 and coil L5. No D. C. voltage is applied to the plates of V1 and V2, but these tubes are plate modu- E lated in push-pull by means of audio frequency voltage supplied by the conventional push-pull audio amplier V7, Va, through modulation transformer T2. T1 indicates the input transformer to this audio amplier stage, and A2, B1 and D1 are the filament, plate and grid current sources, respectively. The double side band output of balanced modulator 3 is induced by coil Ls into coil Ln. Modulator 4 is identical with modulator 3, and audio modulating voltage is supplied thereto by an audio amplifier stage V5, Vs, identical with the stage Vv, Vs, primed reference characters being employed to indicate similar elements in each. The audio frequency inputs to the two audio amplier stages are supplied through phase shift networks 5 and 6, from a common program line I2, so that the audio modulating voltages to the two balanced modulatorsrwill be essentially 90 electrical degrees out `of phase throughout the `useful audio frequency range.

22 and23 respectively. The modulatorsare of In the Abridge circuit .7, 'the outputof balanced modulator 4 is induced from tuned circuitV C5, Ls,

into coil Ls, which is connected to the center tap of coil L1. The two ends of coil L7 are connected 'tocoils L9 and L10, respectively. This connection constitutes a bride arrangement, such that coil Lg Vcarries the sum of the two currents, for example, and coil L10 carries their difference. S1, S2, Ss and Si are electrostatic shieldsplaced between the coils, uas shown, the purpose of which is to enable a high degree of balance to be obtained in the bridge circuits. It is of importance in this application of the bridge principle that the bridge serves not. only to combine two currents, but to combine them in such a way that no radio frequency voltage will be induced in coil L5 as a result of currentin coil Ls, nor will there be any-induced in coil Le as a result of current in coil L5. This is an essential feature of my invention, inasmuch as any inter-action between the two balanced modulator stages wouldv result in` `serious cross-modulation and distortion.

- The single side band current in coil L9 induces Voltage in coil L11, which is applied to the grid of linear amplifier tube V9 through blocking condenser Cs.y Coil L11 -is made selective by condenser Ci, and resistor R2 is an artificial loading resistor for tuned circuit L11, C'z. The purpose of this' additional loading is to prevent the variation in load caused by the periodical flow of grid current in V9 from seriously upsetting the balance of the bridge circuit. In amplifier 8, Ca is a blocking condenser, Hi a radio frequency plate choke, G1 a source of D. C. plate Voltage, H5 a radio frequency grid choke, D2 a grid bias source, A3 a filament current source, and C9 a neutralizing condenser. Coil L12 and condenser C10 constitute the tuned output circuit for the linear amplifier 8 from which single side band Aenergy is induced into coil L13 of bridge circuit I0. Amplifier 9 includes tube V10 in a linear amplifier stage identical with amplifier 8, and primed refer,- ence characters are employed therein to indicate similar elements. Bridge circuit I0 is similar to circuit l, in that it serves to combine the outputs of the two linear amplifier stages so that they add in the branch connected to phase shift `network I I, for example, and subtract in the branch connected directly to antenna circuit 2. Coil L16 is coupled with coil L12 in the output of amplifier 9, and is connected with a center tap on coil L13.

The endsl of coil L13 are connected respectively with the two antenna circuits to complete the bridge arrangement. Coils L14 and L15, and condenser C11 constitute a 90 degree phase shift network, at I I, which may be designed for the carrier frequency, and yet maintain essentially 90 degree phase shift over the comparatively narrow frequency range required.

In the modified form of my invention shown in Fig. 3, the circuit arrangement is generally similar to the corresponding portion of the single side band transmitter of my copending applica-k tion Serial No. 176,670, above noted,` and not basically different in the preliminary circuit elements from the arrangement shown in Fig. 1 of this application. The carrier oscillator and ampliier I and the program line source I2 of modulation energy are the same as in Fig. 1, and the carrier phase shifting network 2 and audio phase shifting networks 5 and 6 effect the same 90 phase displacements characteristic of the same elements in Fig. 1. Ampliers 20 and 2I are provided in the audio frequency channels and feed modulation energy to the balanced modulators the type shown in application Serial No. 176,670,

modulator 23. Blocking condensers C12 and radio frequency choke coils H6 are provided in the grid circuits as indicated, and grid bias vin the moduu-lators is supplied from sources D3 andDi as shown. f I l By the application` of the bridge circuit of my invention to the combination of the doubleside bandv waves in the outputs of both modulators 22 and 23, interaction or cross modulation in the two circuits is prevented. The plate electrodes` in the tubes in each balanced modulator are connected together and to the respective load circuit, 24 or 25, thence in common to the source of plate power G2, through radioy frequency choke ductively coupled toopposite diagonal terminals of bridge circuit 26. Two arms of the bridge -are formed by capacities C13 and C14, a third bythe loadv presented by the ClassvB linear amplifier at 21, and the fourth by a resistor 28 which operates to dissipate -the undesired side bands. Ampliiier 21 receives the superior or the inferior side bands from both balanced modulators 22 and 23, depending upon whether the outputs of the modulators are subtracted or added in the arm of the bridge comprising the input circuit of the amplifier 21. The bridge circuit 26 is balanced so that no interaction between the balanced modulators 22, 23, can occur. The single side band thus derived and amplified at 2'I is fed to the antenna system for radiation.

While I have disclosed my invention in certain preferred embodiments, I do not intendto be limited specically thereto, and it is to be understood that modifications may be made within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as fole lows:

1. In combination with a pair of balanced modulator circuits each delivering double-side band currents, means for dividing the double vside band currents into inferior and superior side band components, means for separately amplifying the inferior and the superior side band components, means for recombining inferior and superior side band components `into a pair of double side band waves of amplified character, and means for shifting the phases of the side band components 'in one of said double side band waves.

`withcarrierand modulation energy impressed in push-pull .relation on the gridsof the tubes, V11

and V12 in modulator 22, .and V13 and V14 in 2. A split side band amplifier for dual double side band circuits comprising a bridge circuit ble side band components therein, separate amv pliiier means connected with adjacent arms of said bridge for independently amplifying the inferior and superior side band components, and a second bridge circuit supplied at both pairs of opposite terminals with single side band energy fromsaid separate amplier circuits respectively, adjacent arms of said second bridge coils H7'. The load circuits. 24 and 25 arevin- 1.20*

.circuit carrying double side `band energy of amvplied character.

3. A split side band system including a bridge circuit, means for supplying said bridge circuit at both pairs of opposite terminals with double side band energy, with carrier and modulation factors respectively in phase quadrature, the double side band energy in adjacent arms of said bridge circuit being additive and subtractive respectively, resulting in the former instance in inferior side band components and `in the latter in superior side band components, of like phase; separate amplifier means connected in said adjacent arms of said bridge circuit, a second bridge circuit, and means for supplying said second bridge circuit at the pairs of opposite terminals with said inferior and said superior side band components respectively, the single side band components in adjacent arms of said second bridge circuit being additive and subtractive respectively, resulting in each instance in double side band energy, carrier and modulation factors kin the resulting double side band energy in said adjacent arms being respectively in phase quadrature. 4. A split side band system including a bridge galones circuit, means for supplying said bridge circuit at both pairs of opposite terminals with double side band energy, with carrier and modulation factors respectively in phase quadrature, the double side band energy in adjacent arms of said i bridge circuit being additive and subtractive respectively, resulting in the former instance in inferior side band components and in the latter `in superior side band components, of like phase;

separate amplier means connected in said adjacent arms of said bridge circuit, a second bridge circuit, means for supplying said second bridge circuit at the pairs of opposite terminals with said inferior and said superior side band components respectively, the single side band components in adjacent arms of said second bridge circuit being additive and subtractive respectively, resulting in each instance in double side band energy, carrier and modulation factors in the resulting double side band energy in said adjacent arms being respectively in phase quadrature, and means connected with one arm of said second bridge circuit for shifting the phase of the carrier factor in the output thereof.

WALTER H. WIRKLER. 

